The present invention relates to a highly strengthened spring suitable for suspension springs and valve springs for automobiles and in other industries, and relates to a production process for the same, and particularly relates to technologies to greatly improve the durability thereof.
Heretofore, in automotive springs typified by suspension springs, the service stress (design stress) has been higher to save energy, and these springs are required to have superior cold creep properties (sag resistance) and high durability. Springs are normally produced in such a way that spring steels are subjected to heat treatments of quenching and tempering after forming. In recent years, a tendency has been seen in which the tempering temperature is lowered to increase the hardness of the springs and to improve cold creep properties. It is known that durability is generally improved by increasing the hardness of the springs by such a method.
However, since the notch sensitivity increases as the hardness of the material increases, the scattering of the durability increases and the reliability of the springs is decreased. When the material is hard, shot used for shot peening processes yields to the hard material. This means that the working by a shot peening process is difficult and a compressive residual stress layer, which is most effective for improving the fatigue strength, cannot be sufficiently imparted, and this therefore results in an important problem in that the fatigue strength cannot be improved. In order to facilitate the performance of the shot peening process, increasing the hardness of the shot may be contemplated. In this case, the shot readily breaks and production cost is relatively high. In such circumstances, much research in shot peening technology for improving durability has been made.
As a shot peening technology to further improve durability, warm shot peening performed at a warm temperature range has been known, as is proposed in Japanese Patent No. 725630. As proposed in U.S. Pat. Nos. 959,801 and 3,094,768, and Japanese Patent Application, First Publication, No. 148537/93, stress shot peening performed while applying stress to spring steels has been known. Moreover, double shot peening is known in which spring steels are subjected to a shot peening process and are then subjected to a shot peening process using smaller shot than in the first process. One-time shot peening cannot increase the compressive residual stress in the outermost surface portion of the spring steel. The double shot peening can increase the compressive residual stress in that portion by causing a plastic deformation there at using a small shot.
However, even these various types of shot peening technology are not sufficient to cope with the recent increased service stress due to the weight-reduction in springs. It is therefore an object of the invention to provide a highly strengthened spring and a production process for the same, which can greatly improve durability in high service stress.
The applicant or assignee previously proposed a warm shot peening method in Japanese Patent Application, First Publication, No. 140643/93 in which the hardness of the spring steel and the temperature in the warm shot peening are limited. The inventors have researched combining the warm shot peening method with the stress shot peening, and they have discovered that the durability of spring steels is greatly improved thereby.
The process for producing highly strengthened springs of the present invention has been made based on the above knowledge. The present invention provides a highly strengthened spring producing process comprising performing a first shot peening on a spring steel having a Vickers hardness (hereinafter referred to as xe2x80x9cHV) of 550 or more (corresponding to a diameter of 2.7 mm or less on a Brinell ball mark, which is hereinafter referred to as xe2x80x9cHBDxe2x80x9d) in applying stress to the spring at a warm temperature in the range of 150 to 350xc2x0 C. The effects and advantages of the invention will be explained hereinafter together with the reasons for the above numerical limitations.
Hardness of Spring Steel: BHD 2.7 mm or More
FIG. 1 shows the relationship between the hardness of the coil spring and the number of cycles to failure when a fatigue test was performed on two types of coil springs which were subjected to shot peening at a warm temperature or at room temperature. In the warm shot peening, the coil spring was heated to 300xc2x0 C. for 20 minutes and was immediately subjected to shot peening. As will be understood from FIG. 1, in the coil spring subjected to the warm shot peening, the durability is improved as the hardness of the coil spring increases, and remarkable improvement of the durability can be obtained when the hardness is 550 HV (HBD 2.7 mm) or more.
Thus, the effects of improvement in durability due to the warm shot peening is sufficiently demonstrated when the hardness of the spring steel is 550 HV or more (HBD 2.7 mm or more). Therefore, the hardness of the spring steel is set at 550 HV or more (HBD 2.7 mm or more). Since the durability is greatly improved when the hardness of the spring steel is 600 HV or more (HBD 2.6 mm or more), the hardness of the spring steel is preferably 600 HV or more. It should be noted that HBD is the diameter of a recess formed by impacting a cemented carbide ball with a diameter of 10 mm against a surface of a sample at a load of 300 kgf.
Temperatures for Warm Stress Shot Peening: 150 to 350xc2x0 C.
FIG. 2 shows the relationship between the temperature of coil springs in shot peening and the number of cycles to failure. As will be understood from FIG. 2, the durability is superior in the overall temperature range of 200 to 350xc2x0 C. in the shot peening than in the case in which the shot peening was performed in room temperature. It can be assumed that the same effect as the above can be obtained at 150xc2x0 C., which is relatively lower than 200xc2x0 C., and the temperature range in the warm shot peening is set in the range of 150 to 350xc2x0 C. The temperature in the warm shot peening is preferably in the range of 250 to 325xc2x0 C. since the durability is most improved in this temperature range. It should be noted that the durability is relatively decreased when the temperature in the shot peening is 350xc2x0 C. The reason of this is that the degree of working by the shot peening is large and the surface roughness is large at 350xc2x0 C. as also shown in FIG. 2, and the notch sensitivity is therefore increased. Furthermore, the relief of the compressive residual stress is promoted at 350xc2x0 C., and this also causes decrease in durability.
The present invention is characterized in performing a first shot peening on a spring while applying stress to the spring at a warm temperature in the range of 150 to 350xc2x0 C. In a conventional stress shot peening while applying stress to the spring at room temperature, it was believed that the stress promotes plastic flow caused by shot, and large compressive residual stress could be generated deeply. However, in the highly strengthened springs according to the invention, the hardness of the springs is equivalent or greater than that of shot. Therefore, sufficient plastic flow cannot be obtained by shot peening by only imparting stress, and sufficient improvement in the fatigue strength cannot be obtained. Therefore, shot peening is performed while applying stress at a warm temperature in the invention. As a result, the inventors have discovered that astonishing improvement in durability can be obtained compared to in warm stress shot peening or shot peening at room temperature.
That is, the highly strengthened spring of the present invention is obtained by performing a first shot peening on a spring steel having a 550 HV or more (corresponding to a diameter of 2.7 mm or less on a Brinell ball mark) while applying stress to the spring at a warm temperature in the range of 150 to 350xc2x0 C., whereby large compressive residual stress is deeply imparted. Therefore, the durability of the spring during use is greatly improved. The preferred embodiments of the invention will be explained hereinafter.